Photonic technology can be used to enhance the measurement and distribution of microwave signals. This field is sometimes called radio-frequency (RF) or microwave photonics. As an example, antenna remoting is a term that refers to modulating a microwave or RF signal onto an optical signal. The modulated optical signal can then be sent over long distances via a fiber optic cable which has lower loss, weight, and cost than an RF cable and does not suffer from electro-magnetic interference, to a receiver that converts the signal back into the electrical domain. Ideally very simple equipment is located at the antenna as the antenna location is chosen for optimal reception and often there may be space or power constraints near the antenna.
Ideally the microwave signal is exactly reproduced at the receiver with no loss of signal integrity. However, usually the signal integrity is limited by nonlinearities in the optical modulator or optical demodulator at the receiver. This creates spurious signals whose magnitude depends on the magnitude of the input RF signal, thereby limiting the dynamic range of operation. This effect is sometimes characterized by the spurious free dynamic range (SFDR) metric. Methods to linearize modulators are often complex, and one does not want complex equipment that may be sensitive to drift or precise parameter settings near the remote antenna. A method to cancel out the third order nonlinear distortion that uses the natural modulation efficiency (characterized by the voltage required to induce a π phase shift, or Vπ) difference of an optical modulator between two different optical wavelengths or the difference between two polarization axes of an optical modulator have been demonstrated. While effective, it turns out such methods also reduce the gain of the systems which in turn reduces the noise figure. This reduces the utility of such linearization methods. A simple method to enhance the SFDR using second harmonic generation (SHG) has been proposed which is compatible with photonic integrated circuit (PIC) integration. Here SHG enhances the effective modulation index of the phase modulator, which allows for the generation of a more nonlinear signal which can be subtracted from the desired signal (which does not undergo SHG) to remove distortion terms.
Another benefit of RF-photonics can be all optical down-conversion, which in principle can replace the electrical mixers more commonly used to down-convert a very high microwave carrier frequency to a lower and more easily detectable carrier frequency. Electrical mixers often have loss, added distortions, and less operating frequency range than desired. Usually these systems are similar to standard heterodyne detection in that they mix in the desired frequency band with a different signal in the image frequency band. The image band can in principle be separated from the desired signal if the signal is measured both in-phase (I) and in quadrature phase (Q). However, such measurements typically require multiple modulators and thus can be inefficient in terms of the electrical power required to down-convert the signal and the number of components required to realize the system. They also may not provide complementary outputs, where complementary outputs allow for noise reduction of optical intensity noise via balanced detection. An optical hybrid has been used to provide for complementary outputs for an optical I/Q down-conversion system, but this system formed a very long interferometer that is not well suited for some applications like optical remoting.
A prior art implementation of an optical I/Q downconverter is shown in FIG. 1. A laser 100 at a wavelength of λ passes through a signal phase modulator 102 which is modulated by the RF signal of interest with a carrier frequency off fRF. The optical output from the signal phase modulator is split by an optical splitter 101 into two arms, one arm is modulated by an in-phase (I) down-conversion (DC) phase modulator 104 and the other arm is modulated by a quadrature phase (Q) DC phase modulator 106. Both DC phase modulators are driven by a LO at frequency fLO, but before the Q phase modulator the LO is shifted in phase by π/2 in an electrical phase shifter 108 which could be realized by a fixed length of RF cable or a 90° electrical hybrid circuit. The optical outputs from the DC phase modulators are filtered using optical filters 110,112 with one optical filter after the I DC phase modulator 110 and one after the Q DC phase modulator 112. The filters can be notch filters or bandpass filters or other types of filters including near-loss-less phase-only filters that convert the signal from pure phase modulation, which is generally not detectable using a direct optical-to-electrical detector, to amplitude modulation which is detectable with direct optical-to-electrical detectors. The filtered signals are detected using photodetectors 114,116. The output of the photodetectors are now the I and Q electrical signals at the IF frequency of fIF=fRF−fLO, assuming fLO<fRF (or otherwise fIF=fLO−fRF). However, they also carry signals at the image band frequency into the signal phase modulator near fRF−2·fIF. By subtracting the I and Q electrical signals with an appropriate phase shift in an I/Q IF 90° hybrid 118 the signal and image frequency can be separated allowing for detection of the desired signal at fRF without the distorting image band. Note that two separate phase modulators are used in this design, each having a separate hot electrode carrying the LO signal.
A method of creating a microwave photonic link that makes use of a shared electrode in the signal modulator offers some benefits including the ability to match the frequency response of the two effective modulators. The two modulators modulate the RF signal onto different optical paths with different modulation index, thereby allowing for subtraction of some nonlinear distortions. The known design uses amplitude modulators and does not perform down-conversion. Amplitude modulators are usually based on the Mach-Zehnder interferometer (MZI) configuration, which is an interference-based device and requires careful setting or knowledge of the inherent phase bias to achieve high dynamic range. In antenna remoting applications it is more desirable to use a simple phase modulator that does not require a phase bias. Such phase-modulation based systems have been demonstrated, including linearized systems that employ down-conversion.
Optical modulators such as Mach-Zehnder Interferometer (MZI) style Lithium Niobate (LN) modulators sometimes operate in the “push-pull” configuration where a common hot electrode applies a phase shift of opposite sign to two arms of an interferometer. In an interferometer these two arms are combined optically to convert the phase modulation into an intensity modulation.
What is needed is a system or method to efficiently down-convert and measure microwave signals over a photonic link. The RF signal should be imparted using a simple phase modulator avoiding for instance the need for setting a phase bias voltage. It should preserve a high dynamic range and allow the possibility of noise reduction via balanced detection. It should be capable of generating both in-phase and quadrature phase signals in a simple configuration that is compatible with photonic integrated circuit (PIC) integration. The number of independent hot electrodes should also be minimized to simplify the system and reduce or eliminate the requirement for electrical splitters to distribute the LO to multiple electrodes. The system should also make maximal use of resources, including the ability to down-convert multiple input signals using a single down-converter.